In general, tape transport systems in which the supply and take-up reels are simultaneously driven by surface engagement of a capstan with the outermost layer of tape on each reel require the presence of tension in the tape span between the supply and take-up reels. The tension permits the formation of a tape pack wound on a flangeless hub which will retain its physical shape without side support and also will withstand the high speeds and rapid accelerations and decelerations normally associated with this type of unit.
Some tape transports of the general type described above also require constant tension to assure uniform recording headtape contact to enable the transduction of a flutter-free signal. An example of such a type transport is disclosed in pending U.S. Application No. 388,929, now U.S. Pat. 3,921,933 filed Aug. 16, 1973, the disclosure of which is incorporated herein by reference. The tape tension in that tape transport is maintained almost entirely by braking the reel serving as the supply reel.
Known tape transports have generally relied almost exclusively on a differential deformation phenomenon. An example of such a tape transport is set forth in U.S. Pat. No. 3,370,803. By using a resilient capstan and applying a greater contact force between the take-up reel and the capstan than between the supply reel and the capstan, the resulting differential in the deformation of the capstan periphery at the two interfaces maintains tension in the tape between the supply and the take-up reels.
Each of the two tension-producing/maintaining methods have advantages and disadvantages when relied upon as the sole means of achieving the requisite constant tape tension.
Utilization of the differential deformation effect has the distinct advantage of minimal power consumption. Relying solely on the differential deformation efffect is subject to certain disadvantages. For one thing, in a tape transport relying solely on the differential deformation effect, tape loops frequently develop, especially between the body of the tape pack on the supply side and the zone of interface between the tape and the capstan on the supply side. Unless the loops are straightened out, the transport action fails. Such tape loops may occur, for example, if a new pack which has not previously been passed through the transport mechanism is placed on the supply side of the apparatus or if a pack prior to its use in the transport apparatus has been in storage for a considerable length of time and for this reason has become "soft".
In such cases the required pretension level in the tape is not achieved until after the first several passes through the transport. It is during the period before which an adequate tension level is achieved that the tape loops are most likely to develop.
Another drawback of a tape drive relying on the differential deformation principle for tensioning the tape is that such a drive exhibits considerable periodic variations of the tape tension due to "run out". Any run out of the capstan, or of the reels -- or more generally, the corresponding "satellite" assemblies -- which is caused by tolerances in the concentricity of the hubs, tape packs or bearings, creates a change in the radial force at the interfaces between the capstan and the satellites. Also, any changes--due to manufacturing variations, wear, flat spots, etc. --in the hardness of the resilient material on the capstan (rubber tire) result in tape tension variations, and therefore flutter in the transduced signal.
As discussed above the generation of stable compressive forces of either of the two satellites against the capstan is of great importance in providing a reliable transport having low flutter and small variations in tape tension. Thus tape drive utilizing the above-mentioned differential deformation principle is particularly sensitive to the change in the compressive force between the capstan and the satellites--for the reason that in such a drive the tape tension is generated by the difference between these forces at the two interfaces.
There is a disadvantage in known tape transports which in relying solely on the differential deformation phenomenon to provide tape tension maintain a constant force differential between the supply reel and take-up reel interfaces with the capstan. It has been found that under such conditions a variation in capstan deformation occurs as the size of the reel of tape changes. Tests surprisingly indicate that given a constant contact force, a larger tape reel will produce less capstan deformation than a smaller reel. As the amount of deformation changes the tension produced by such deformation likewise changes. Therefore, in order to maintain constant tension, varying forces instead of constant forces must be employed to urge the supply and take-up reels against the capstan.
Above referenced Co-pending United States Application Ser. No. 388,929 discloses a tape transport which avoids many of the disadvantages described above by relying upon supply reel braking. The system uses a pair of unidirectional brakes, one on each reel shaft, to resist the driving of the capstan when the reel is serving as the supply reel but to allow unrestricted rotation when the reel is serving as a take-up reel.
The forces urging the supply and take-up reels toward the capstan are substantially equal to avoid any significant differential deformation and the aforementioned problems associated therewith. The problem of tape loops encountered with loosely wrapped tape packs is substantially eliminated as are the problems encountered with run out. The unidirectional brakes have a varying braking characteristic which produces a substantially constant resisting force at the point of driving contact between the supply reel and the capstan regardless of the size of the supply reel. This non-constant feature results in a nearly constant tension profile in the resulting take-up tape pack.
A major disadvantage of the system relying solely on the braking of the supply reel to maintain tension, however, is that more power is required to drive a capstan which in turn must drive a braked supply reel.
As can be seen, reliance upon either one of the two tension-producing/maintaining means has serious disadvantages. A combination of the two effects can eliminate the respective disadvantages to a certain extent. Known systems combining the two effects either have introduced new disadvantages or have not succeeded in sufficiently eliminating the disadvantages enumerated above.
For example, it is known through U.S. Pat. No. 3,638,880 to provide tape cartridges with unidirectional brakes to eliminate tape loop problems encountered when using loosely packed reels with a mechanism of the type disclosed in the earlier referenced U.S. Pat. No. 3,370,803. The technique is expensive, however, since it requires each cartridge to be equipped with a unidirectional brake.
In addition, the brake is a constant torque device the effect of which is to increase the force required to drive the supply reel at its periphery as the supply reel decreases in size during the tape transporting operation. Such an increase in the driving force can result in a decreasing capstan driving speed due to the increased of work load on the drive motor. Even slight tape speed variations are highly undesirable in video tape recording playback systems and must be avoided.
A tape transport system utilizing unidirectional compensating brakes for reel size changes in conjunction with the differential deformation principle is disclosed in U.S. Pat. No. 3,482,800. In that system, however, the brakes only restrict the rotation of the supply reel enough to maintain a constant work load on the capstan motor in order to maintain a uniform capstan speed as the size of the supply reel varies. Separate torque motors produce the differential deformation which is the predominating tension maintaining means.
This system overcomes the disadvantage of that disclosed in U.S. Pat. No. 3,638,880 but as the brakes are not employed primarily to create tape tension but instead only to restrict the rotation of the supply reel enough to maintain a constant work load on the capstan motor as the size of the supply reel varies, the problem of component run out and its effect on tension maintenance through the differential deformation effect is still present.
In addition none of the known tape transports relying either solely or primarily on the differential deformation effect compensate for the significant variation in differential deformation which occurs due to changing reel sizes as explained above.